Any caustic drain cleaner can damage the wax ring seal between the toilet and the flange. They're not dificult to replace, however. You'll know if you did, if water starts running out on the floor around the base of the toilet.
The most likely place for a clog is in the built-in toilet trap. Almost anything can clog a toilet trap (too much paper, plastic toys, toothbrushes, feminine products, etc.).
If you can't get the caustic drain cleaner to go down, I would carefully dip it out of the bowl into a bucket and pour it down another clear drain.
Refill the bowl from a bucket of water, and try a toilet plunger.
If that doesn't clear it, buy or rent a crank-handle toilet auger (made to clear toilet traps). That will usually dislodge whatever is clogging it.
If that doesn't work, you may have to pull the toilet and clear the tra from underneath.
Good luck!
Mike